Fixes are listed in file FIXES.md

+ fix the link to full changelog

This causes a crash of uv-qt on U21.10.

Actually it suffices to remove libffi.so.6 instead but it is a
dependency of SDL2.

Bundle i965-va-driver-shaders instead of i965-va-driver. This should be
superior:

"This package contains also contains the non-free encode shaders for
VP8, VP9, HEVC and AVC for generation 7.5 hardware or newer."

Fixed broken JPEG produced by current FFmpeg if slices > 1 and
threads = 1.

 Fixed SVT-VP9 patch not applying - we need more repo depth now
 (increased 1000->5000 commits - 2000 would take 18 MiB, 5000 24 MiB,
 which is stil decent).

Use non-linear gradient for the background to avoid sharp borders of the
background visible with on finer displays.

Do not add dependency/build hwaccel_libav_common.o if libavcodec is not
present as a dependency for OpenGL display.

- "gradient" is prefix of "gradient2" and the first was checked earlier
  thus gradient2 was never selected
- incorrect check for boundaries with min instead of max

Small clarifications/cleanups

Enabled remaining 3-/4-bit ANSI colors as per:
https://en.wikipedia.org/wiki/ANSI_escape_code#3-bit_and_4-bit

rang.hpp header is now synced with:
https://github.com/MartinPulec/rang/blob/master/include/rang.hpp

[decklink] fix stereo 3D supportedFlags

This partially reverts 2a511d58. The value 4 should be default now (best
quality), BPS==2 should be perhaps set later by a preset (after/if
implemented).

Added stub allowing dummy decode (without error correction) for the
receiver basic run.

+ do not abort on unknown PT in audio

Clarify that PT_JPEG isn't our JPEG but RFC-compliant.

Support FPS values that are not understood by UG (FPSd == 0) and
replace it by 30.

This is a forward compatibility feature when there may be special meaning FPS
values (unlimited etc.) but it can still be displayed, especially by displays
that don't honor FPS value like OpenGL.

- fixed [,] special meaning in M4
- qmake version >6 -> >=6
- typo: QMAKE_VER instead of QMAKE_VERSION

For Linux AppImages in release channel, zsync files are stored as an
artefact in continuous release assets so ensure that it is present.

Cargo 1.56 is needed for cargo-c but 1.55 is in U18 runner ver
20211017.0 thus update as a workaround.

Failed run:
https://github.com/MartinPulec/UltraGrid/actions/runs/1412342915

As there will be releases 1.7.1, 1.7.2 etc., storing zsync in release
asset doesn't make sense (further version won't update previous update
information). Thus store this in continuous release assets - update
channel "release" was introduced aside to existing "continuous".

The release channel is not build from release/ branch, instead a bugfix
version numbers are required and the build is from appropriate tag, eg.
v1.7 (release, should be in master) and then v1.7.1, v1.7.2. The bugfix
releases should have its own entry in NEWS file (create-release.sh makes
GitHub release description from this).